1. Field of the Invention
The present invention relates to a surface acoustic wave (hereinafter, referred to as “SAW”) duplexer including a SAW resonator, for use in, for example, cellular phones, and relates to a communication device having the same.
2. Description of the Related Art
Recently, in small wireless communication apparatuses, such as cellular phones, a duplexer has been used for branching and sorting transmission/reception signals. In view of the demand for miniaturization, reduced weight, and sophistication of the communication apparatus, the components of the duplexer are required to be miniaturized and sophisticated as well. This has led to most recent filters using SAW elements.
As such a filter, a ladder filter obtained by combining SAW resonators into a ladder configuration is used. Such a ladder filter has an advantage in that it can decrease insertion loss. In Japanese Unexamined Patent Application Publication No. 5-167388 (a known example 1), there is disclosed a circuit construction in which, as shown in FIG. 10, the above-described ladder filter is used both in the transmission side and reception side to provide a duplexer construction.
Furthermore, in Japanese Unexamined Patent Application Publication No. 9-167937 (a known example 2), there is disclosed a surface acoustic wave filter in which, as shown in FIG. 11, an inductor L82 is connected in parallel with a series resonator 41 at the output side of the ladder filter to realize great attenuation over the out-of-pass-band.
However, the construction of the above-described known example (1) is a common ladder filter without any particular feature for achieving characteristic improvement. This causes the attenuation to be determined by the capacitance ratio of the series resonators to the parallel resonators, which has a tradeoff relationship between the attenuation and the loss. Therefore, the known example (1) has a loss deterioration problem, particularly in the duplexer required for high attenuation.
In the known example (2) is disclosed the construction in which the parallel inductor is connected to a single unit of the ladder filter. The construction is, however, assumed exclusively for the single unit of the ladder filter, and when it is used in the duplexer, there is no consideration of the characteristic deterioration thereof. Particularly in the known example (2), there are disclosed, as a method to form the inductor, examples in which the inductors are formed using a micro-strip line and/or a bonding wire disposed on a SAW chip that makes the ladder filter. In the duplexer having two different frequency pass-bands with the inductors thus formed, mutual interference cannot be ignored, causing deterioration in attenuation and isolation.